This invention relates to hot melt inks containing dispersed pigments and, more particularly, to a pigmented hot melt ink in which the pigment dispersion is effectively stabilized.
Pigmented hot melt inks have significant advantages over hot melt inks containing dyes. One advantage is that pigments tend to be more light-fast than dyes, which is an important factor in connection with the storage of printed images. In addition, pigments tend to be more heat-stable than dyes and this is especially important when the ink is a hot melt ink applied at an elevated temperature of, for example, 80.degree.-150.degree. C. Furthermore, pigments are less likely than dyes to bleed or sublime and they may be either opaque or transparent, whereas dyes are transparent and have little covering power.
Heretofore, however, dyes have usually been preferred for use in hot melt inks, particularly for ink jet inks, since they are completely dissolved in the medium and cannot settle whereas, pigments being suspended and dispersed, may settle to the bottom of a reservoir or agglomerate to block ink jet ink nozzles.
In inks containing a liquid vehicle such as water or oil, pigments such as carbon black, phthalocyanine blue, lithol rubine and diarylide yellow have previously been used and surface-active materials or the like have been added to such inks in order to wet the pigment and break agglomerates of primary pigment particles, disperse the pigment to develop color strength, and stabilize the pigment from settling or agglomerating. In hot melt inks such as those containing hydrocarbon waxes and polymers which are used at elevated temperatures, it has been found that such surface-active materials do not provide adequate particle wetting or dispersion stability. Among the additives used in liquid-vehicle inks which have been found to be essentially ineffective as pigment dispersants and pigment stabilizers in hot melt inks are alkyd polymers, materials containing amphipathic molecules such as Solsperses.RTM., lecithins, alkylated polyvinyl pyrrolidones, metal soaps, stearic acid, paraffin wax, polyethylene wax, carnauba wax, candelilla wax, montan wax, hydrogenated castor oil, microcrystalline wax, behenic acid, aluminum stearate, synthetic ester waxes, oxidized polyethylene waxes, lauric acid, Fischer-Tropsch waxes, esparto waxes, dammar wax, oxazoline wax, bisamide waxes, amide waxes, oleamides, stearamides, lauramides, erucamide, glycerol monostearate, glycerol tristearate, chlorinated waxes, cetyl alcohol, stearone, laurone, diphenyl phthalate, dicyclohexyl phthalate, camphor, triphenyl phosphate, n-ethyl toluene sulphonamide, n-cyclohexyl-p toluene sulphonamide and other natural and synthetic polymers and resins.
Moreover, even if such prior art additives were effective, many of them would not be useful in hot melt inks because they tend to degrade or evaporate at high temperature or to impart an undesired color to the ink or are otherwise incompatible with the ingredients of a hot melt ink. Furthermore, where hot melt inks are intended for use in an ink jet printer, for example, the physical properties of the ink must be carefully controlled to assure proper operation of the printer. Consequently, any additive which may affect the physical properties of the ink is undesirable. Also, pigmented hot melt inks tend to flocculate or agglomerate after extended exposure to high temperature and high shear stress to which they are subjected in a hot melt ink jet system.